1. Field of the Invention
The present invention relates to a lithography exposure technique and, more particularly, to a technique which utilizes spatial filtering in the Fourier transform plane of the mask to provide twice the resolution of conventional lithography systems.
2. Description of the Prior Art
Various types of integrated optical circuits are beginning to require the formation of sub-micron sized features. For example, distributed feedback lasers include an internal grating layer with a period of 0.23 .mu.m (feature size 0.115 .mu.m), and distributed Bragg reflectors on dielectric waveguides often require a period of 0.5 .mu.m (0.25 .mu.m feature size).
The ability to provide such fine-line feature resolution has been primarily relegated to two techniques: e-beam direct writing and holographic lithography. E-beam systems, although capable of providing resolution of less than 0.1 .mu.m, have several drawbacks. First, the process must be performed in a vacuum, requiring that the wafer be placed in a chamber, the chamber evacuated, and the exposure completed. Further, since the electron beam actually "writes" each desired line (or feature), the time required to completely transfer a mask pattern may be quite lengthy. Thus, the throughput time for each wafer becomes excessively long when compared with processes using other exposure techniques, leading to an increase in cost for each wafer produced. Another cost factor is related to the equipment necessary to perform this type of operation. As a result, e-beam direct writing lithography is not considered to be the method of choise for a high volume, industry environment.
The holographic lithography technique mentioned above is also capable of exposing line widths in the 0.25-0.12 .mu.m range. However, this system is limited in that it can produce only a uniform intensity profile at the wafer surface. That is, holographic techniques can expose only uniform gratings. Thus, this technique is difficult, if not impossible, to employ when it is desired to expose a non-uniform pattern (i.e., chirped or phase-shifted gratings); particularly, the gratings of different periods and shapes placed at different locations on the chip. Additionally, holographic systems, in general, tend to be relatively expensive and are not considered a viable alternative for factory utilization.
In light of the above, much research has been directed toward improving the resolution of photolithography systems for use in the sub-micron regime. One such sub-micron photolithography system is disclosed in U.S. Pat. No. 4,450,358 issued to G. O. Reynolds on May 22, 1984. The Reynolds' system utilizes a deep UV source (excimer laser), coherent condenser optics, a reflective optical system and course/fine focus control, and is capable of exposing line widths in the range of 0.25-0.50 .mu.m. However, the focus control, as described, requires a significant increase in both the system cost and the time required to perform an exposure.
An alternative sub-micron photolithography technique is disclosed in U.S. Pat. No. 4,360,586 issued to D. C. Flanders et al. on Nov. 23, 1982, and relates to a spatial period division technique. In particular, a mask having a spatial period p is separated from the surface of the wafer by a distance S determined by the relation S=p.sup.2 /n.lambda.. For n=2, the near-field diffraction of a light source through the mask will result in doubling the period of the mask on the wafer surface (n=3 will triple). Therefore, the period of the grating exposed on the wafer will be a function of the gap separating the mask from the wafer. Problems arise, however, in the ability to control this gap so as to provide consistency and uniformity from wafer to wafer for the same mask. For example, with .lambda.=0.25 .mu.m (KrF excimer laser) and p=0.5 .mu.m, S should be 0.5 .mu.m, which is difficult to control. For some systems, excessively long exposure times (&gt;190 hours) are also required. A subsequent paper on this same subject, by A. M. Hawryluk et al., entitled "Deep-ultraviolet spatial-period division using an excimer laser", appearing in Optics Letters, vol. 7, No. 9, Sept. 1982, pp. 402-4, addresses the long exposure problem. By using an excimer laser source, Hawryluk et al. found that exposure times could be reduced to under one hour (25 minutes, for example). However, the laser-based system still relies on the mask-to-wafer gap control for correct exposure.
Yet another system is described in an article entitled "Excimer laser-based lithography: a deep ultraviolet wafer stepper" by V. Pol et al. appearing in SPIE, Vol. 663, "Optical Microlithography V", Mar. 1986 at pp. 6-16. Here, a deep UV projection system was developed by modifying a commercial step and repeat exposure system to operate at 248 nm with an all-quartz lens and a KrF excimer laser. The quartz lens exhibits a 5.times. reduction and has a field size of 14.5 mm, with a numerical aperture (NA) which is variable from 0.2 to 0.38. This system has been found to produce a resolution of 0.5 .mu.m over the 14.5 nm field, with a depth of focus in the range of approximately .+-.2 .mu.m. However, the contrast exhibited by this system is too low to provide the printing of gratings with a smaller period, for example, 0.4-0.5 .mu.m (0.2-0.25 .mu.m feature size).
Thus, a need remains in the prior art for a lithographic technique which is capable of sub-half-micron resolution, relatively inexpensive and easy to implement, and capable of providing consistent exposures.